This invention relates to an apparatus for determining the time at which a charged droplet is at a predetermined physical position. The invention finds application in the measurement of the velocity of ink droplets generated by ink jet printing systems.
U.S. Pat. No. 4,417,256, discloses an apparatus comprising an electrode past which the charged droplet passes in use of the apparatus. Circuitry responsive to the charge induced on the electrode by the passing droplet, determines the time at which the droplet is in the neighbourhood of the electrode.
The apparatus of U.S. Pat. No. 4,417,256 does not precisely correlate the position of the droplet with the time at which the droplet is at that specific position. In particular it is not possible to say quite what point on the current waveform which arises due to the charge induced on the electrode, corresponds to the time at which the droplet is at a specific physical position in space with respect to the electrode.
According to a first aspect of the present invention there is provided an apparatus for determining the time at which a charged droplet is at a predetermined physical position comprising: first and second electrodes past which said droplet passes in use of said apparatus, said droplet inducing a charge on each said electrode as it passes; and circuitry responsive to the charges induced on the first and second electrodes for determining the time at which said droplet is at said predetermined physical position midway between said electrodes.
Preferably, the circuitry comprises: a differential amplifier, each of said first and second electrodes being connected to a respective one of the inputs to the differential amplifier; and a zero crossing detector for detecting the zero crossing between adjacent pulses of opposite sign output by said differential amplifier in response to the charges induced on the electrodes by the passing charged droplet.
According to a second aspect of the present invention there is provided an apparatus for measuring the velocity of a charged ink droplet generated by an ink jet printing system comprising: first and second spaced pairs of electrodes past which said droplet passes in use of said apparatus, said droplet inducing a charge on each said electrode of the pairs as it passes; and circuitry responsive to the charges induced on the electrodes for determining the time at which said droplet is midway between the first pair of electrodes and the time at which the droplet is midway between the second pair of electrodes, the velocity measurement being provided by dividing the distance between these two midway points by the time between the times at which the droplet is at these two midway points.
Preferably, said circuitry comprises: a differential amplifier, the first electrode of each pair passed by the droplet being connected to one input of the differential amplifier, the second electrode of each pair passed by the droplet being connected to the other input of the amplifier; and a zero crossing detector for detecting the zero crossing between adjacent pulses of opposite sign output by the amplifier in response to the charges induced on the pairs of electrodes by the passing charged droplet.